Soap bubbles to take the drag out of future cars

Soap bubbles filled with helium are helping to improve the fuel efficiency of future cars.

The 3-millimetre bubbles swirl around cars in a wind tunnel. Engineers at automotive research consultants Mira in Nuneaton, UK, use 12 cameras to track the bubbles, and so capture air flows in unprecedented detail (see video, above).

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The helium in the bubbles gives them neutral buoyancy&colon; left to their own devices they will neither rise or fall in the air, so any up or down movement can be attributed to air flow around the car.

“There aren’t any tools in use today that can give such insight into what’s going on in the fluid around a vehicle,” aerodynamics specialist Angus Lock, who is leading development of the system, told New Scientist.

Low drag

Consumers are beginning to consider fuel economy and carbon emissions when choosing a new car, says Lock, and so aerodynamics has become much more important to car manufacturers. Cutting a vehicle’s air resistance is usually a cheaper way of improving those stats than reworking an entire engine or drivetrain.

For example, although the Toyota Prius’s hybrid engine helps boost its mileage, so does the car’s shape, which creates much less drag than other cars in its class.

The bubble technique is not completely new. It has long been used to see how air moves around a structure&colon; for example, it was used to test models of NASA’s space shuttle. But Mira’s camera system gives extra insight by capturing the precise movement of individual bubbles in 3D for later analysis and exploration.

Simulation quality

Full-scale wind tunnel tests remain the gold standard for looking at a car’s aerodynamics, Lock says, and it’s simple to try out new ideas for improving streamlining by simply swapping parts of the car in the tunnel. Sensors measure how the car interacts with the air rushing past.

“But they can’t visualise the whole flow field around the vehicle,” says Lock, “so you don’t know what is behind those forces.” The bubble-tracking approach captures the speed as well as the direction of air flow, which makes it more useful than existing tracking techniques such as injecting smoke trails around the car.

Movie cameras

The infrared Vicon camera system that Mira uses is more usually found inside game or movie studios to capture the movements of actors wearing reflective markers, or in labs that study human motion.

The Mira team push the cameras to their sensitivity limits to track light reflected from the bubbles instead. “We are thinking about what we can do to the bubbles to make them easier for the cameras to see,” Lock says.

Building a machine able to produce larger bubbles – which would have to be filled with a mixture of helium and air to achieve neutral buoyancy – is one idea the team are considering. They also plan a machine that can produce more bubbles.

Despite the increasing sophistication of computer simulations, finding ways to show complex air flows visually is critical to understanding aerodynamics, says Alex Liberzon at Tel Aviv University in Israel, and new ways to do that in large wind tunnels are valuable.

“You cannot solve everything completely in space and time on a computer,” Liberzon told New Scientist. “Simulations do not capture the full complexity of wakes and other features, which can exhibit large changes in behaviour caused by very small changes.”